Ignore specific files and folders publishing to NPM

Users of `scratch-vm` do not need `playground` or `test` directories
installed as a npm package. If they need those directories they can
clone scratch-vm and do a local build. This saves downloading 42MB
(`playground`) and 4.4MB (`test`) when `npm install`ing `scratch-vm`.

.npmignore

@@ -1,2 +1,20 @@
+# Development files
+.eslintrc.js
+/.editorconfig
+/.eslintignore
+/.gitattributes
+/.github
+/.travis.yml
+/.tx
+/test
+/webpack.config.js
+
+# Build created files
+/playground
+
+# Coverage created files
 /.nyc_output
 /coverage
+
+# Exclude already built packages from testing with npm pack
+/scratch-vm-*.{tar,tgz}